This invention relates to novel methods and apparatus for improving the high speed performance of rotary wing aircraft.
There are two types of heavier than air aircraft that achieve lift by movement through the air:
(1) The airplane, which has stationary wings that create lift when propelled through the air by a thrust mechanism such as a propeller or jet engine, and
(2) The rotorcraft or rotary wing aircraft in which blades rotate to describe a disc above the aircraft to create lift.
There are three types of rotorcraft that utilize a blade to provide lift:
(1) The helicopter, in which the rotor blade provides vertical thrust and, because the rotor disc can be tilted on a supporting and rotating vertical mast, a horizontal thrust component.
(2) The autogyro, in which lift is provided by a rotary wing and forward thrust provided by a propeller or a jet. Autogyration is achieved by tilting the rotor disc back relative to the airflow so that some air flows up between the blades and through the rotor disc rather than down through the rotor disc as in a helicopter. As the air flows up through the rotor disc, the rotor is driven much like a windmill is driven by the wind.
(3) The gyroplane, described in U.S. Pat. No. 5,727,754, in which a rotor is used for vertical and slow speed flight, but at high speed cruise the rotor is unloaded (provides almost no lift) and the wing provides nearly all the lift.
Juan de la Cierva in Spain invented the autogyro in 1923. Successful autogyros were produced in England and by several companies in the U. S., with Pitcairn being the most notable manufacturer. In the 1930""s autgyro technologywas rapidly advancing and its safety and utility were being demonstrated and accepted. Mail carrying autogyros operated from the top of the Philadelphia Post Office. Four- and five-passenger autogyros were being produced as well as smaller ones. Pitcairn alone developed and manufactured 14 models between 1930 and 1940. These aircraft had performance equaling contemporary airplanes with maximum speeds up to 150 mph.
The quest for faster rotorcraft has been ongoing ever since. One basic problem is that a rotor""s lift is limited by the lift that can be produced by the retreating blade, since the aircraft will roll if the total lift moments on the advancing blade and retreating blade are not equal. At high aircraft forward speeds, the retreating blade tends to stall and lose lift, because the rotor RPM cannot be increased without the advancing blade tip going faster than the speed of sound. Because of this problem, the ratio of aircraft forward speed to rotor tip speed, known as Mu, is limited to about 0.5 in helicopters and autogyros.
To achieve the highest speed flight with a gyroplane or helicopter it is necessary to reduce rotor lift during horizontal flight, to reduce the problems with retreating blade stall. This goal is disclosed in U.S. Pat. No. 3,155,341, issued to The Ryan Aeronautical Company, Nov. 3, 1964. The English Frairey Rotodyne, which had a wing and tip jet autorotating rotor, used for take off and landing, set a closed course speed record for rotorcraft of 191 mph in 1959. The Russian KAMOV KA-22 broke this speed record in 1961 with a speed of 221 mph. The current record is approximately 250 MPH. All these aircraft reduce lift on the rotor by having some lift provided by a wing or by providing auxiliary thrust with a separate engine so that the rotor provides lift but no thrust. However, none of them exceed a Mu of 0.5.
The drag of a rotor blade increases with the cube of the rotation rate. Therefore, it is a great advantage if the rotation rate can be reduced. The ratio of aircraft forward speed to rotor tip speed, known as Mu, must be increased as much as possible, probably over 1.0. The challenge, then, is to maintain autorotation and rotor stability at high Mu. This is the subject of this invention.
It is the general object of the invention to provide an improved method of operating rotorcraft capable of achieving high speeds.
In general, this object is achieved by varying collective pitch, including to negative values, to maintain acceptable levels of flapping at high aircraft forward speeds and low rotor rotation rates, or adjusting or maintaining the rotor rotation rate by automatically controlling the tilt of the rotor disk relative to the airstream or aircraft, or a combination of these techniques. More specifically, by utilizing these techniques the forward aircraft speeds can be high enough, and the rotor rotation rates low enough, that an advance ratio, Mu, greater than 0.75 can be achieved while maintaining rotor stability.